The present invention relates to an image forming apparatus in which image data is processed specially and a plurality of laser beams scans one pixel so that the image of high density and sharpness can be reproduced.
In the field of image forming apparatus using an electrophotographic method, a digital halftone image is reproduced in the following manner: an image signal of an original image is read by a scanner; and image density data in which the image signal is gradation-corrected, A/D converted, and shading-corrected, is modulated by a reference signal.
When an original image is read by the scanner, an edge portion of the image is read in a halftone density due to the aperture of a solid state image pick-up element installed in the scanner. When a latent image is formed on a photoreceptor with image density data obtained from this image signal, recording pixels corresponding to the edge portion of the latent image are recorded on average in the case where the density is intermediate. Therefore, sharpness of the recorded image is lowered. The aforementioned deterioration of sharpness can not be prevented even when MTF correction is conducted on the image signal.
On the other hand, even when an interpolated character or figure is formed from computer graphic (C.G.) data or font data, the same problem is caused. That is, when the edge portion is interpolated smoothly by the intermediate density of the interpolation data, a recording pixel corresponding to the edge portion is recorded in pixels as average density, and thereby the resolution is lowered in the same manner.
As shown in FIG. 19, most of conventional data is developed to a binary bit-map. Therefore, the edge portions of characters do not become smooth. Especially when variable magnification RE or MTF correction is conducted, zig-zags edges appear or some portions of a character are cut out. For that reason, it is necessary to conduct intermediate density processing on the edge portion of an image.